1. Field of the Invention
The present invention is generally related to speech coding and, more particularly, to methods and systems for realizing scalable speech codecs with fine grain scalability (FGS) in a CELP-type (Code Excited Linear Predictive) coder.
2. Background
The flexibility of bandwidth usage in a transmission channel has become a major issue in recent multimedia developments, where the amount of data and number of users occupying the channel are often unknown at the time of encoding. Multi-bit-rate source coding is one of the solutions. In accordance with this type of coding, a scalable source codec apparatus with FGS, which requires only one set of encoding algorithms while allowing the channel and a decoder the freedom to discard various numbers of bits in the bit-stream, has become favored in the next generation of communication standards.
For example, general audio and video coding algorithms with FGS have been adopted as part of MPEG-4, which is the international standard (ISO/IEC 14496). The FGS algorithms used in MPEG-4 general audio and video share a common strategy, in that the enhancement layers are distinguished by the different bit significance level at which a bit plane or a bit array is sliced from the spectral residual. The enhancement layers are so ordered that those containing less important information are placed closer to the end of the bit-stream. Therefore, when the length of the bit-stream to be transmitted is shortened, those enhancement layers at the end of the bit-stream, i.e., with the least bit significance levels, will be discarded first.
FGS, although being implemented for audio and video, is not yet applied to speech. This method as it is may not work well for a highly parametric codec with high compression rate (in other words, low bit rate transmission), such as CELP-based ITU-T G.729, G.723.1, and GSM (Global System for Mobile communications) speech codecs. These speech codecs all use LPC-filtered (Linear Predictive Coding) pulses for compensating the residual signals. Due to this difference in coding structure between the CELP algorithms and the MPEG-4 audio and video coding, a CELP-based FGS speech codec has not been fully developed.